jeremyevans/sequel — comment on commit 0202abc1a7564e3a26f2402c7f6c7251d08be918
File:  line 0
By jeremyevans:

Three-level qualifications are occasionally useful, but supporting them directly in symbols would be invasive and hurt performance. Considering how few people need them, I don't think it's worth adding support for them.

You can use the qualify method if you want to really need a three level qualified identifier. In the case you describe, you can already handle that easily by giving the schema qualified table an alias so that columns of that table don't need to have a three level quailfied identifier:

DB[:schema1__table1___t1].cross_join(:schema2__table2___t2).where(:t2__column2=>:t1__column1)